The channels encountered by many wireless communication systems often scatter the transmitted signal along its transmission path. Time variation of the channel results in random fluctuations of the received power level, or fading, making reliable communications difficult.
Transmitters typically employ channel coding techniques that map sequences of input data to codewords that add redundancy to combat the effects of fading and noise prior to transmission. Codewords consist of a number of symbols carrying data at the transmission rate, the number of information bits communicated with each symbol. The channel coherence time is the amount of time the time-varying channel is assumed constant; signals transmitted within the coherence time are affected by a single fading state. During transmission, each codeword is affected by one or more fading states with the specific number affecting the communications performance The coding delay is proportional to the codeword length and is often quantified in terms of the number of fading states affecting each codeword; it significantly affects a system's reliable communications performance. A system is considered delay unconstrained if it uses infinite-length codewords resulting in infinite coding delays Practical communication systems are delay-limited; they use finite-length codewords and therefore have a finite coding delay.
Conventional analysis of fading channels has been performed from the single-attempt paradigm. That is, the amount of information that can be reliably communicated with a single codeword transmission attempt has been quantified. This approach works well for idealized, delay-unconstrained systems that transmit a single, infinite-length codeword. However, practical systems are delay-limited since they use finite-length codewords. Therefore, the conventional performance metrics based on the single-attempt paradigm have drawbacks for delay-limited systems: ε-capacity—the highest transmission rate that can be supported with a probability of data loss no greater than ε—does not provide a measure of error-free performance, while single-attempt delay-limited capacity—ε-capacity when data loss cannot be tolerated; that is, when ε=0—underestimates achievable performance.